Piezoelectric resonators are commonly used in the frequency control circuitry of electronic devices. Their low cost, size, weight, and power consumption when used with appropriate excitation circuitry, make them the devices of choice for many applications, particularly for mobile systems. One of their limitations is, however, the long time frequency drift known as resonator aging. In applications where ultra-low aging is required, high cost, high power atomic standards must be employed. The use of atomic standards also imposes weight and reliability limitations. As a result, reduction of the aging rate of piezoelectric resonators represents a significant need for the frequency control community.
It is commonly believed that one of the major causes of aging in piezoelectric resonators is mass transfer due to residual contamination remaining in the resonator enclosure at the completion of fabrication. Past attempts at reducing the aging caused this contamination have generally involved improved clean procedures and sealing tecnniques. However, there is always some small amount of residual contamination within the enclosure no matter how good the cleaning process. A means of stabilizing the residual contamination is therefore desired as a means of reducing the resonator aging.
It is also well known that the frequency of a piezoelectric resonator is sensitive to the mass loading on its major surface. Contamination adsorbing or desorbing from these major surfaces will contribute to undesired frequency shifts. From kinetic theory, the average residence time of a molecule on a surface is given by ##EQU1## where T is the temperature, R is the gas constant, and .tau. is approximately 10.sup.-13 seconds, and Ed is the desporption energy of the contaminating species. In the past, workers have taken advantage of the temperature dependence of the residence time to remove contamination by high temperature vacuum baking prior to sealing, and it is commonly believed that nothing can be done to influence the residual contamination in the sealed enclosure.